Continuous polymeric liner production methods for conformable pressure vessels

ABSTRACT

A method and apparatus for forming a pressure vessel liner are disclosed. One method includes forming an extruded tube by extruding a parison through a die and a mandrel and forming main body sections and return line sections in the extruded tube according to a first pattern. A cross-sectional area of the return line sections is smaller than a cross-sectional area of the main body sections. The method further includes changing the pattern according to which the main body sections and the return line sections are formed from the first pattern to a second pattern without stopping the forming of the extruded tube and forming the main body sections and the return line sections in the extruded tube according to the second pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 62/411,918, titled “Continuous Polymeric Liner ProductionMethods for Conformable Pressure Vessels,” filed Oct. 24, 2016, theentirety of which is incorporated herein.

BACKGROUND

Conformable pressure vessel concepts offer improved storage efficienciesand reduced system weights compared to traditional cylindrical designs.These pressure vessels may be comprised of a plurality of smallercylindrical vessels that can be stacked or bundled to fit into apredetermined packaging space.

Production of conformable pressure vessels requires the fabrication of apolymeric pressure vessel liner to act as a barrier layer to preventpermeation of the liquids and gasses contained by the pressure vessel athigh pressures. One method for production of pressure vessel liners isto extrude a continuous plastic tube and send the plastic tube through acorrugation process that creates the desired profile comprising rigidmain body sections and flexible return lines.

One limitation of existing fabrication methods is insufficient speed ofthe corrugator, which restricts production capacity. Fabricating thelarger main body sections and smaller return lines requires speedreductions in the corrugator, further restricting production volumes.Producing larger amounts of plastic pressure vessel liners thereforerequires numerous pieces of expensive production equipment andsignificant floor space.

An additional limitation of existing equipment is that the number ofmold elements in the repeating pattern is constrained by the equipmentdesign. Some pressure vessel liner geometries are only possible toachieve by adding an excessive amount of additional mold elements to thepattern. This lengthens the equipment, which adds to the cost andoverall size of the equipment. Other pressure vessel liner geometriesare not possible to achieve because the pattern of mold elementsrequired would be too large to be practical.

An additional fabrication method for pressure vessel liners involves theseparate production of extruded main body sections and formed returnlines. This improves throughput, but joining the segments togetherintroduces several potential leak paths that are exacerbated by highpressure, extreme temperatures, and pressure cycling.

Another limitation of existing fabrication methods is inconsistentmechanical properties, including variability of wall thickness andmaterial properties of the pressure vessel liners. This is caused by thelimitations of the forming equipment which has difficulties creating alarger outer diameter and a smaller return line diameter from a singleextruded tube.

Another limitation of existing fabrication methods is the inflexibilityof the forming equipment, which cannot produce flexible main bodylengths without stopping the process and reconfiguring the equipment.Stopping production and reconfiguring equipment further reduceseffective production capacity and results in more connections betweenvarying length pressure vessels, leading to additional potential leakpaths.

SUMMARY

Several alternative polymeric pressure vessel liner production methodsand apparatuses are disclosed. The methods include improving theflexibility of pressure vessel production by introducing automatedmethods for changing out mold elements without stopping the productionrun. Alternative forming methods are also disclosed for producingvariable lengths of main body sections with improved mechanicalproperties at high extrusion rates without the need for reconfigurationof a corrugator.

One method for forming a pressure vessel liner includes forming anextruded tube by extruding a parison through a die and a mandrel;forming main body sections and return line sections in the extruded tubeaccording to a first pattern, wherein a cross-sectional area of thereturn line sections is smaller than a cross-sectional area of the mainbody sections; changing the pattern according to which the main bodysections and the return line sections are formed from the first patternto a second pattern without stopping the forming of the extruded tube;and forming the main body sections and the return line sections in theextruded tube according to the second pattern.

One apparatus for forming pressure vessel liners includes an extruderthat drives a parison through a die and a mandrel to form an extrudedtube; and a formation unit that forms the extruded tube into main bodysections and return line sections according to a first pattern, changesthe pattern according to which the main body sections and the returnline sections are formed to a second pattern without stopping theextrusion of the extruded tube; and forms the extruded tube into themain body sections and the return line sections according to the secondpattern.

Another method for forming a pressure vessel liner includes forming anextruded tube by extruding a parison through a die and a mandrel; usinga corrugator having a set of mold elements, forming main body sectionsand return line sections in the extruded tube according to a pattern;using an interchange mechanism, selectively replacing large diametermold elements in the set of mold elements with small diameter moldelements; and using the corrugator and the selectively replaced moldelements, forming the main body sections and the return line sections inthe extruded tube according to the pattern.

Another method for forming a pressure vessel liner includes forming anextruded tube by extruding a parison through a die and a mandrel; andforming main body sections and return line sections in the extruded tubeby alternating application of small diameter sections and large diametersections of a pair of drums to the extruded tube. A cross-sectional areaof the return line sections is smaller than a cross-sectional area ofthe main body sections, and a pattern of the main body sections and thereturn line sections is formed without stopping the forming of theextruded tube by applying the pair of drums to form the small diametersections on the extruded tube.

Another method for forming a pressure vessel liner includes forming anextruded tube by extruding a parison through a die and a mandrel; andforming main body sections and return line sections in the extruded tubeby clamping opposing sections of a molding press together onto theextruded tube. A cross-sectional area of the return line sections issmaller than a cross-sectional area of the main body sections. A patternof the main body sections and the return line sections is formed withoutstopping the forming of the extruded tube by expanding the opposingsections of the molding press in a longitudinal direction along an axisparallel to a longitudinal axis of the extruded tube or controlling afrequency with which the opposing sections press together onto theextruded tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Itis emphasized that, according to common practice, the various featuresof the drawings are not to-scale. On the contrary, the dimensions of thevarious features are arbitrarily expanded or reduced for clarity.

FIG. 1 is a side view of an extrusion corrugator for production ofconformable pressure vessel plastic liners.

FIG. 2 is a top view of corrugator.

FIG. 3 shows a portion of the corrugator of FIG. 2 with a vacuum andheating box attached.

FIG. 4 is a side cut away view of a corrugation mold element showingvacuum slots.

FIG. 5 is a detail view of vacuum slots in mold elements passing by avacuum tube.

FIG. 6 is a top view of corrugator showing an automated mold elementselection and replacement apparatus.

FIG. 7 is a side view of a top-loading automated mold element selectionand replacement apparatus.

FIG. 8 is a side view of extrusion equipment with a molding press.

FIG. 9 is an axial view of molding press of FIG. 8.

FIG. 10 is a side view of an expanding molding press.

FIG. 11 is a side view of an expanding molding press and variable speedextrusion pulling rollers.

FIG. 12 is a top view of a pair of opposing drums.

FIG. 13 is a cross-sectional view of the small diameter section of thepair of opposing drums.

FIG. 14 is a cross-sectional view of the large diameter section of thepair of opposing drums.

DETAILED DESCRIPTION

Pressure vessel liners can be formed using various types of formationunits that includes components such as corrugators, mold presses, and/oropposed drums used to shape an extruded plastic parison. Flexibility inapplication of these various components can improve production speed,reduce space within a production facility, and allow flexibility inparison type and grade when compared to a more standard corrugationprocess.

FIG. 1 is a side view of a corrugator 100 for production of pressurevessel liners for conformable pressure vessels. The corrugator 100 maybe positioned in either a vertical or a horizontal manner. Thecorrugator 100 provides a means to form corrugated and/or straightsections of continuous tubing 102 to serve as the pressure vesselliners. A plastic parison of a predetermined weight is first extrudedthrough an appropriately sized die and mandrel. The predetermined weightof plastic per unit of length is that is necessary to produce thedesired wall thickness in the pressure vessel liner. For example, apressure vessel liner 104 can have a wall thickness determined by thevessel design. The corrugator 100 can include a corrugation bed 106containing circulating pairs of mold elements 108 in a set pattern usedto produce the pressure vessel liners.

FIG. 2 is a top view of a corrugator 200. The circulating pairs of moldelements 202 are shown, using arrows to indicate the direction ofrotation. Application of the mold elements 202 produces a continuous andrepeating pattern of main body sections 204 having a largecross-sectional area and return line sections 206 having a smallercross-sectional area along the pressure vessel liner. Depending on thelength of the main body sections 204, each rotational cycle of the moldelements 202 can produce one or more of the main body sections 204. Thelength of a corrugation bed 208 can be extended by adding additionalmold elements 202 and by adding spacers between motor spindles 210 thatdrive the corrugator 200. This extension in length is necessary toproduce variants in length for the main body sections 204 of thepressure vessel liner. However, the pattern of mold elements 202 cannotbe adjusted during a production run of the corrugator 200. In otherwords, the pattern of mold elements 202 cannot be adjusted withoutstopping extrusion of an extruded tube 212 through an extruder.Additionally, some lengths of the main body sections 204 cannot beproduced practically because the number of mold elements 202 required toproduce a repeating pattern of main body sections 204 and return linesections 206 of the desired length would exceed the capacity of thecorrugator 200.

FIG. 3 shows a portion of a corrugator 300 with a vacuum and heating box302 attached. The vacuum and heating box 302, also described as a vacuumand heating section 302, is located where molding of an extruded tubeinto a desired pressure vessel liner profile occurs. In the vacuum andheating box 302, vacuum suction can be added to supplement air pressureapplied within the extruded tube in order for the extruded tube to fullyconform to mold elements 304. Additionally, heating through induction orby forced heated air can be used to create sufficient mold surfacetemperatures to prevent such manufacturing issues as inconsistent orpremature cooling of the extruded tube, the extruded tube sticking tothe surfaces of the mold elements, or insufficient molding temperature.

FIG. 4 is a side cut away view of a mold element 400 showing vacuumslots 402. The mold element 400 is fabricated in pairs to form a profileof the outside diameter of each section of the pressure vessel liner.The mold element 400 pairs are separated, indexed, and assembled into acorrugator such that, as the corrugator runs, each mold element 400rejoins its pair as an extruded tube is molded into the pressure vesselliner profile.

FIG. 5 is a detail view of vacuum slots 500 in mold elements 502 passingby a vacuum tube 504. The vacuum tube 504 applies suction pressure tothe vacuum slots 500 in each of the mold elements 502 as they pass byduring each circulation cycle (e.g., the cycle described in respect toFIG. 2). The vacuum tube 504 and the vacuum slots 500 are temporarilysealed with a rolling or sliding seal 506 to preserve the suctionpressure. Using a similar method, heating or cooling can be applied byforcing high temperature air through the vacuum slots 500 in the moldelements 502.

FIG. 6 is a top view of the corrugator 600 showing an automated moldelement selection and replacement apparatus. The corrugator 600 caninclude a first and a second mold element applicator 602, 604. Each ofthe mold element applicators 602, 604 can include a motor spindle 606and a belt 608 coupled to the motor spindle 606 that rotates about themotor spindle 606. Each of the mold element applicators 602, 604 canfurther include a pattern of active mold elements 610 and inactive moldelements 612 attached to the belt 608, wherein the active mold elements610 are applied to an extruded tube 614 to form the main body sections616 and the return line sections 618 and the inactive mold elements 612are not applied to the extruded tube 614. The first mold elementapplicator 602 can be located on a side of the extruded tube 614opposite the second mold element applicator 604. The apparatus can alsoinclude a mold element storage container 620 that holds alternative moldelements 622. An interchange mechanism 624 can selectively remove theinactive mold elements 612 from the first and second mold elementapplicators 602, 604, remove the alternative mold elements 622 from themold element storage container 620, and replace the removed inactivemold elements 612 from the first and second mold element applicators602, 604 with the alternative mold elements 622.

Operation of the corrugator 600 can include applying circulating pairsof active mold elements 610 in a first pattern to the extruded tube 614,where the pairs of active mold elements 610 form a profile of an outsidegeometry of each section of a pressure vessel liner 626. Operation ofthe corrugator 600 can also include removing pairs of inactive moldelements 612 forming the first pattern from the corrugator 600 andreplacing the removed pairs of inactive mold elements 612 with pairs ofalternative mold elements 618 to form a second pattern. Circulatingpairs of the alternative mold elements 618 can be applied according tothe second pattern to the extruded tube 614.

With this apparatus, the mold elements 610, 612 for the corrugator 600can be selectively removed and replaced with the alternative moldelements 618, enabling the pattern of mold elements 610, 612 to becontrolled and varied. For example, since the mold elements 610, 612rotate between positions in which the active mold elements 610 areapplied to the extruded tube 614 to form the main body sections 616 andthe return line sections 618 and in which the inactive mold elements 612are not applied to the extruded tube 614, the alternative mold elements618 can be swapped for particular inactive mold elements 612. Thisenables the same corrugator 600 to produce pressure vessel liners 626with a variety of lengths for the main body sections 616 and the returnline sections 618. Automated mold element 610, 612 selection andreplacement also allows forming a variety of patterns of repeating orvariable main body sections 616 and return line sections 618 without thecorrugator 600 being taken off-line for manual reconfiguration,improving process timing.

FIG. 7 is a side view of a top-loading automated mold element selectionand replacement apparatus 700. In this system, a single or multiplepredefined patterns of mold elements 702 are inserted into a belt ofmold elements as shown in FIG. 6.

The embodiments of FIG. 6 and FIG. 7 enable the fabrication of a varietyof lengths for the main body sections 616 along the same pressure vesselliner 626 within the same continuous production run of the corrugator600. Additionally, unique geometries for the main body sections 616 andthe return line sections 618 can be formed at any point in the pattern,such as at the beginning and end of pressure vessel liners 626 tosupport installation of end fittings. For example, the pairs ofalternative mold elements 622 used in the second pattern can form adifferent profile of the outside geometry of each section of thepressure vessel liner 626 relative to the profile formed by the pairs ofmold elements 610, 612 used in the first pattern.

FIG. 8 is a side view of a molding press 802. Opposing sections 804 ofthe molding press 802 clamp together onto an extruded tube 806 to formalternative geometries onto an otherwise uniformly shaped extruded tube806. The molding press 802 can have the opposing sections 804 where eachof the opposing sections 804 is shaped such that the main body sections808 and the return line sections 810 are formed when the opposingsections 804 are clamped together onto the extruded tube 806. Operationof the molding press 802 can include clamping the opposing sections 804of the molding press 802 together onto the extruded tube 806 andreleasing the opposing sections 804 from the extruded tube 806 accordingto a first pattern. Operation of the molding press 802 can also includechanging a frequency with which the opposing sections 804 clamp togetheronto the extruded tube 806 to form a second a pattern. The opposingsections 804 of the molding press 802 can be clamped together onto theextruded tube 806 and released from the extruded tube 806 according tothe second pattern. In other implementations, operation of the moldingpress 802 can also include expanding the opposing sections 804 of themolding press 802 in a longitudinal direction along an axis parallel toa longitudinal axis of the extruded tube 806 when the opposing sections804 of the molding press 802 are clamped onto the extruded tube 806.

The molding press 802 can be horizontally stationary with the opposingsections 804 moving up and down to clamp around the extruded tube 806.The molding press 802 can also be configured to translate horizontallyat the same speed as the extruded tube 806 to avoid breaking, tearing,or bunching up of the extruded tube 806 proximate to the molding press802. In other words, in some implementations the molding press 802 canbe stationary relative to longitudinal motion of the extruded tube 806and in other implementations the molding press 802 can move along anaxis parallel to a longitudinal axis of the extruded tube 806 at a speedat which the extruded tube 806 is extruded.

Lead-in features in the horizontally axial direction may also be used toavoid abrupt geometry changes. The molding press 802 may alsoincorporate heating and vacuum features similar to those described inrespect to FIGS. 3, 4, and 5. The application of a single or multiplemolding presses 802 can be programmed to produce a variable pattern ofcross-sectional geometries as required by the final design of thepressure vessel liner. This enables pressure vessel liners to bemanufactured that include multiple main body sections 808 and returnline sections 810 without the need to join various sections of separatepressure vessel liners together, reducing the opportunity for leaks inthe pressure vessel liner. Additionally, the speed of the extrusion canbe maximized by producing a single cross-section and does not need to bevaried to produce the different sections of the pressure vessel liner.

FIG. 9 is an axial view of a molding press 900. Production of main bodysections and return line sections for pressure vessel liners can beeffected using the molding press 900. Opposing sections 902 of themolding press 900 can be applied directly to an extruded tube 904 toform main body sections, return line sections, or other uniquegeometries. In one implementation, the return line sections are formedby narrowing ends of each of the main body sections of the pressurevessel liner, making the return line sections more flexible than themain body sections.

FIG. 10 is a side view of an expanding molding press 1000. The expandingmolding press 1000 can have opposing sections 1002, where the opposingsections 1002 clamp together onto an extruded tube 1004 and expand in alongitudinal direction along an axis parallel to a longitudinal axis ofthe extruded tube 1004 to form main body sections 1006 and return linesections 1008. The expanding molding press 1000 can move along an axisparallel to the longitudinal axis of the extruded tube 1004 at a speedat which the extruded tube 1004 is extruded.

The expanding molding press 1000 expands the extruded tube 1004 in atargeted region in order to reduce the specific weight of material, forexample in regions of the return line sections 1008, in order to reducewall thickness and enable improved material properties. The expandingmolding press 1000 can be fitted with a progressive die, for example, aspring-loaded die, that guides the extruded tube 1004 into a propershape at the selected region. The progressive die can be connected tothe expanding molding press 1000, where the progressive die forms theextruded tube 1004 into the main body sections 1006 and the return linesections 1008 in a region of the extruded tube 1004 where the opposingsections 1002 of the expanding molding press 1000 clamp together. Theexpanding molding press 1000 can also be fitted with cutters that can beused to trim off excess material after shaping.

FIG. 11 is a side view of a molding press 1100 and variable speedpulling rollers 1102. Operation of the expanding molding press 1100 caninclude stretching an extruded tube 1104 in the longitudinal directionwith the variable speed pulling rollers 1102 rotating about an axisperpendicular to the longitudinal axis of the extruded tube 1104 whenthe opposing sections 1106 of the molding press 1100 are clamped ontothe extruded tube 1104. The variable speed pulling rollers 1102 canstretch the extruded tube 1104 to reduce the specific weight of materialmaking up the extruded tube 1104. While opposing sections 1106 of themolding press 1100 are moved vertically to clamp around the extrudedtube 1104, the opposing sections 1106 may also be translatedhorizontally to move at the same speed as the extrusion to avoidbreaking, tearing, or bunching up of the extruded tube 1104. Lead-infeatures in the horizontally axial direction may also be used to avoidabrupt geometry changes. The molding press 1100 of this example may alsoincorporate heating and vacuum features as described in FIGS. 3, 4, and5.

FIG. 12 is a top view of a pair of opposing drums, or drums 1200 a,b.The extruded tube 1202 can extend between the drums 1200 a,b, where eachdrum 1200 a, 1200 b rotates about an axis perpendicular to alongitudinal axis of the extruded tube 1202. Each drum 1200 a, 1200 bcan include an outside radial surface having a small diameter section1204, a cross-section of which marked by location A-A is shown in FIG.13, and a large diameter section 1206, a cross-section of which markedby location A-A is shown in FIG. 14, where the drums 1200 a,b form themain body sections 1208 and the return line sections 1210 by applyingthe small diameter sections 1204 of the drums 1200 a,b to the extrudedtube 1202.

In some implementations, as the extruded tube 1202 passes through thedrums 1200 a,b, the drums 1200 a,b are rotated to the large diametersection 1206 to allow the extruded tube 1202 to pass through the drums1200 a,b without touching either of the drums 1200 a,b. When it is timeto form a return line section 1210, the drums 1200 a,b are rotated suchthat a small diameter section 1204 of the drums 1200 a,b forms the mainbody sections 1208 and the return line sections 1210 as the extrudedtube 1202 and the drums 1200 a,b move together. In some implementations,the drums 1200 a,b can be stationary relative to longitudinal motion ofthe extruded tube 1202. In other implementations, the drums 1200 a,b canmove along an axis parallel to a longitudinal axis of the extruded tube1202 at a speed at which the extruded tube 1202 is extruded. This canallow the extruded tube 1202 to be created with varied lengths of mainbody sections 1208 having return line sections 1210 in between.

Operation of the drums 1200 a,b can include moving the extruded tube1202 in a longitudinal direction through the drums 1200 a,b where anoutside radial surface of each drum 1200 a, 1200 b has a small diametersection 1204 and a large diameter section 1206. The operation of thedrums 1200 a,b can also include rotating each of the opposing drums 1200a, 1200 b about an axis perpendicular to a longitudinal axis of theextruded tube 1202 and forming the main body sections 1208 and thereturn line sections 1210 into the extruded tube 1202 by applying thesmall diameter sections 1204 of each opposing drum to the extruded tube1202 according to a first pattern.

A frequency with which the small diameter sections 1204 of each opposingdrum 1200 a, 1200 b are applied to the extruded tube 1202 can be changedto form a second a pattern. Operation of the drums 1200 a,b can alsoinclude forming the main body sections 1208 and the return line sections1210 by applying the small diameter sections 1204 of each opposing drum1200 a, 1200 b to the extruded tube 1202 according to the secondpattern. In some implementations, the operation of the drums 1200 a,bcan include changing speed of the extruded tube 1202 relative to thedrums 1200 a,b before and after the small diameter sections 1204 of thedrums 1200 a,b are applied to the extruded tube 1202 to change a wallthickness of the extruded tube 1202. In other words, the extrusion speedand relative motion of the extruded tube 1202 before and after theextruded tube 1202 passes through the drums 1200 a,b can be manipulatedto ensure the proper wall thickness is produced in the return linesections 1210. In some implementations, multiple drum pairs can be usedto produce multiple different shapes on the same extruded tube. Forexample, one set of drums can form the ends of an extruded tube sectionwithout corrugations to enable the attachment of end fittings, andanother set can form return line sections.

The embodiments of FIGS. 10-14 enable faster production speeds, improvematerial properties, allow the use of a wider variety of materialgrades, and allow vastly more flexibility than can be achieved using amore standard corrugation process.

While the disclosure has been described in connection with certainembodiments, it is to be understood that the disclosure is not to belimited to the disclosed embodiments but, on the contrary, is intendedto cover various modifications and equivalent arrangements.

What is claimed is:
 1. A method for forming a pressure vessel liner,comprising: forming an extruded tube by extruding a parison through adie and a mandrel of an extruder; circulating a corrugator by rotatingmotor spindles interfaced with a pair of tracks, wherein the pair oftracks each have a fixed length around the motor spindles and each havecirculating pairs of mold elements; forming main body sections andreturn line sections in the extruded tube according to a first patternby applying the circulating pairs of mold elements positioned on thepair of tracks to the extruded tube, wherein a cross-sectional area ofthe return line sections is smaller than a cross-sectional area of themain body sections; changing the pattern according to which the mainbody sections and the return line sections are formed from the firstpattern to a second pattern; exchanging the circulating pairs of moldelements with circulating pairs of alternative mold elements by using aninterchange mechanism without stopping the circulation of the corrugatorby the rotating motor spindles and without stopping the forming of theextruded tube by the extruder; and forming the main body sections andthe return line sections in the extruded tube according to the secondpattern by applying the circulating pairs of alternative mold elementsin the second pattern to the extruded tube.
 2. The method of claim 1,wherein exchanging the circulating pairs of mold elements with thecirculating pairs of alternative mold elements by using the interchangemechanism includes: removing all pairs of mold elements in thecirculating pair of mold elements that form the first pattern from thecorrugator; and replacing the removed pairs of mold elements with pairsof alternative mold elements to arrange the circulating pairs ofalternative mold elements in the second pattern so that the fixed lengthof the pair of tracks remains constant when the pairs of mold elementsare replaced by the pairs of the alternative mold elements.
 3. Themethod of claim 2, wherein the pairs of mold elements form a profile ofan outside geometry of each section of the pressure vessel liner, andwherein the pairs of alternative mold elements in the second patternform a different profile of the outside geometry of each section of thepressure vessel liner relative to the profile formed by the pairs ofmold elements in the first pattern.
 4. The method of claim 1, whereinexchanging the pair of circulating mold elements with the pair ofcirculating alternate mold elements by using the interchange mechanismincludes: selecting a first pair of mold elements included in thecirculating pairs of mold elements; removing the first pair of moldelements using arms of the interchange mechanism associated with thepair of tracks as the corrugator circulates; and replacing the firstpair of mold elements with a second pair of mold elements using the armsof the interchange mechanism to form the circulating pair of alternativemold elements.
 5. The method of claim 4, wherein removing the first pairof mold elements using arms of the interchange mechanism associated withthe pair of tracks as the corrugator circulates includes removing thefirst pair of mold elements using a first pair of arms, and whereinreplacing the first pair of mold elements with a second pair of moldelements using the arms of the interchange mechanism to form thecirculating pair of alternative mold elements includes using a secondpair of arms to add the second pair of mold elements, the second pair ofarms being different than the first pair of arms.
 6. The method of claim4, wherein replacing the first pair of mold elements with a second pairof mold elements using the arms of the interchange mechanism to form thecirculating pair of alternative mold elements includes: moving the firstpair of mold elements from the pair of tracks to a storage containerusing a first pair of arms; and moving the second pair of mold elementsfrom the storage container to the pair of tracks using a second pair ofarms.
 7. A method for forming a pressure vessel liner, comprising: usingan extruder, forming an extruded tube by extruding a parison through adie and a mandrel; using a formation unit including a corrugator havinga set of mold elements, forming main body sections and return linesections in the extruded tube according to a pattern; while the mainbody sections and return line sections are formed according to thepattern and using an interchange mechanism, selectively replacing largediameter mold elements in the set of mold elements with small diametermold elements, comprising: removing the large diameter mold elementsfrom the set of mold elements using arms of the interchange mechanism;and adding the small diameter mold elements into the set of moldelements using the arms of the interchange mechanism; and using thecorrugator and the selectively replaced mold elements, forming the mainbody sections and the return line sections in the extruded tubeaccording to the pattern, wherein a length of the mold elements and alength of the selectively replaced mold elements are equal so that theformation unit run continuously and reduces processing time.
 8. Themethod of claim 7, wherein forming main body sections and return linesections in the extruded tube according to a pattern includes rotatingmotor spindles interfaced with tracks of the corrugator, wherein thetracks of the corrugator are interfaced with the set of mold elements.9. The method of claim 7, wherein removing the large diameter moldelements from the set of mold elements using arms of the interchangemechanism includes rotating motor spindles and tracks of the corrugator,and wherein adding the small diameter mold elements into the set of moldelements using the arms of the interchange mechanism includes rotatingthe motor spindles and the tracks of the corrugator.
 10. A method forforming a pressure vessel liner, comprising: forming an extruded tube byextruding a parison through a die and a mandrel of an extruder; formingmain body sections and return line sections in the extruded tube byalternating application of small diameter sections and large diametersections of a pair of drums in a formation unit to the extruded tube ina first pattern; changing the pattern according to which the main bodysections and the return line sections are formed from the first patternto a second pattern; moving the pair of drums in the formation unitalong an axis parallel to a longitudinal axis of the extruded tube at aspeed at which the extruded tube is extruded in accordance with thesecond pattern; and forming main body sections and return line sectionsin the extruded tube by alternating application of small diametersections and large diameter sections of the pair of drums onto theextruded tube in accordance with the second pattern, wherein across-sectional area of the return line sections is smaller than across-sectional area of the main body sections, wherein a pattern of themain body sections and the return line sections is formed withoutstopping the forming of the extruded tube by applying the pair of drumsto form the small diameter sections on the extruded tube, and wherein athickness of walls of the small diameter sections and a thickness ofwalls of the large diameter sections is controlled by a speed of theextruded tube and a speed of the pair of drums.
 11. A method for forminga pressure vessel liner, comprising: forming an extruded tube byextruding a parison through a die and a mandrel of an extruder; formingmain body sections and return line sections in the extruded tube byclamping opposing sections of a molding press in a formation unittogether onto the extruded tube in a first pattern; changing the patternaccording to which the main body sections and the return line sectionsare formed from the first pattern to a second pattern; moving themolding press in a longitudinal direction along an axis parallel to alongitudinal axis of the extruded tube at a speed at which the extrudedtube is extruded in accordance with the second pattern; and forming mainbody sections and return line sections in the extruded tube by clampingthe opposing sections of the molding press in the formation unittogether onto the extruded tube in the second pattern, wherein across-sectional area of the return line sections is smaller than across-sectional area of the main body sections, wherein a specificweight of the return line sections is larger than a specific weight ofthe main body sections, and wherein a pattern of the main body sectionsand the return line sections is formed without stopping the forming ofthe extruded tube by expanding the opposing sections of the moldingpress in a longitudinal direction along an axis parallel to alongitudinal axis of the extruded tube or controlling a frequency withwhich the opposing sections press together onto the extruded tube.